From my U.S. Pat. No. 4,675,721 (incorporated by reference thereto) there is known a method in which the pulses (code-elements) occurring in PAM or PCM are coded by the amplitudes of half-waves or periods of an alternating current, which are transmitted in an uninterrupted sequence of positive and negative half-waves. If is further known, to unite additively two code-alternating-currents of this type, whose phases are displaced by 90.degree. with respect to each other, and then to transmit the combined oscillation which arises thereby.
In another known method, which is known as quadrature amplitude modulation (QAM), the code symbols of a code alphabet are represented by distinct combinations of amplitude- and phase-values.
It is further known from European patent application 110,421 to reduce the frequency of a code-alternating current, e.g. by one fourth, by taking samples in the time-multiplex-method with 90.degree. phase displacement and transmitting upon the half-waves or periods of four code alternating currents displaced in phase by 90.degree. with respect to each other, of which then two can again be added. The frequency of these four code alternating currents is then only one quarter of the frequency of the original single code alternating current.